1. Field of the Invention
The present invention is directed to a method and system for telecommunications and, in particular, for dispersions maps in long haul and ultra-long haul wavelength division multiplexed optical fiber systems with enhanced distributed gain and/or remotely pumped erbium-based amplification.
2. Related Art
Over the past decade, long-haul data transmission capacity has greatly expanded. This capacity expansion is due to a series of technological developments including erbium doped fiber amplifiers (EDFAs), high speed time division multiplexing (TDM), wavelength division multiplexing (WDM), and non-zero dispersion-shifted optical fiber.
However, transmission capacity is ultimately limited by the interplay of transmission impairments (i.e., the degradation of fidelity of the optical data carrier signal) caused by several fundamental physical phenomena, including attenuation, Rayleigh scattering, dispersion, and optical nonlinearity of the fiber. Each of these known impairments will now be discussed.
Attenuation: Though the glass used for optical fiber is highly transparent at the wavelengths of radiation used for optical data transmission (about 1250 nm to 1650 nm), fundamental physical processes, such as Rayleigh scattering and Erbach-tail absorption, can cause an exponential decay as a function of fiber length in the energy per bit of an optical data signal. This attenuation is generally greater than 0.15 dB/km for even the most nearly ideal silica-based optical fiber. In the absence of any mechanism for reamplifying the optical signal, this attenuation would limit data transmission to a maximum distance of about 500 km or less. The development of optical amplifiers, such as the EDFA, have enabled transmission over much longer distances by periodically boosting the optical signal to overcome the attenuation.
However, amplifiers can introduce noise in the form of amplified spontaneous emission (ASE) which degrades the optical signal. The degree to which the amplification degrades the signal is determined by the physical properties of the amplifier and also by the total attenuation of the signal prior to amplification, i.e., the distance between amplifiers.
Conventional erbium doped fiber amplifiers (EDFAs) operate, for example, in either the “C-band” (conventional band, at about 1530 nm-1560 nm (±10 nm)) or “L-band” (long wavelength, at about 1570 nm-1605 nm (±10 nm)). These conventional EDFAs consist of one or more short (e.g., 10-100 m) segments of optical fiber, the core of which is doped with moderately high concentrations of Er3+ (about 100-1000 parts per million by weight of the oxide (ppmbw)). Conventionally, the fiber will be contained in a single small package (e.g., 2×10×15 cm) along with a pump laser (either 980 nm or ˜1480 nm pump wavelength), and conventional ancillary components (couplers, power converters, monitor and control electronics, etc.) Such amplifiers (having such short segments of doped fiber) are referred to as “discrete EDFAs”. Alternatives also exist, which include amplifiers comprising erbium-ytterbium co-doped fiber.
Discrete EDFAs have been conventionally used to extend the length of so-called repeaterless (or unrepeatered) transmission links. A link is said to be repeaterless if it includes no electrically powered equipment, other than at the terminals (the endpoints of the link). Such links are advantageous where electrical power is difficult or expensive to obtain locally, but is readily available at the terminals. It is advantageous for short-distance (e.g., <600 km) undersea links, as it obviates the need for high voltage power feed equipment at the terminals, and can simplify the design of the transmission cable. However, the attenuation of an ideal optical fiber is >0.15 dB/km, and limits the practical length of such a span with no in-line amplification to about 200-500 km (depending on the channel and aggregate data rates). Discrete EDFAs as described above thus require at least some electrical power to drive the pump laser(s).
An alternative to EDFAs is to use distributed amplification. For example, distributed Raman amplification (DRA) involves launching an optical pump signal, along with the data signal (conventionally counter-propagating) into the fiber composing the transmission span. The wavelength and power (or intensity) of the pump signal is selected to induce stimulated Raman scattering (SRS) within the fiber, so as to amplify the data signal. Contrary to the case for an EDFA, which is essentially a discrete device, the amplification based on SRS may be arranged to be distributed over a large fraction of the transmission span between repeaters.
For example, FIG. 1 shows a plot of relative signal power (in dB) as a function of distance (km) for EDFA versus DRA. FIG. 1 compares the evolution of the signal power over a typical repeater span distance (about 75 km) for transmission with EDFA based (discrete) repeaters and DRA. The degradation of the data signal by ASE noise will be greater in the EDFA case as the maximum loss [minimum signal power] is greater [less] than for the DRA case.
Dispersion: The speed of light (as measured in group velocity) in a material such as silica optical fiber varies significantly with the particular wavelength of the optical signal. This phenomenon is known as group velocity dispersion (GVD, or also referred to as group delay dispersion, GDD). This GVD affects transmission of an optical data signal as the signal must be comprised of a band of wavelengths in order to carry information.
For example, a pulse of light representing an isolated “1” bit will be composed of wavelengths with a spectral bandwidth approximately equal to the inverse of the temporal duration of the pulse. After propagation over a full transmission link, if the total group delay for the shortest wavelengths differs from the delay for the longest wavelengths by more than about one bit period, then a significant fraction (e.g., >25%) of the energy for that “1” bit will spill over into the time slots of neighboring “0” bits. This “spill-over” results in inter-symbol interference (ISI), whereby the values of the “1” bit and its neighbors may be determined erroneously at the terminus of the transmission link (such as at the receiver of a conventional transmission span).
For ideal linear transmission (i.e., neglecting the nonlinear impairments described below), the ISI may be eliminated by arranging for the total dispersion of the transmission link to be essentially zero. To this end, optical fibers have been developed with very low dispersion in the wavelength range of interest, such as dispersion shifted fibers (DSF). However, this particular approach has proven disadvantageous due to nonlinear optical effects.
Alternatively, dispersion compensating fibers (DCF) have been developed to cancel the dispersion of standard single-mode fibers (SMF, i.e., fibers with a zero-dispersion wavelength of about 1310 nm). Transmission spans have also been developed consisting of so-called non-zero dispersion shifted fibers (NZD). For this approach, two types of fibers are alternated in the link. The two fibers are similar in design to the DSF fibers, but with small, non-zero dispersions in the bands of the alternating wavelengths approximately equal in magnitude but of opposite sign; the magnitude of the dispersions are midway between the SMF and DSF fibers.
Nonlinearity: Another important class of transmission impairments results from optical Kerr nonlinearity. This nonlinear effect occurs because the index of refraction of the silica fiber transmission medium depends on the intensity of the light being transmitted through the fiber. For a multi-channel transmission system, where the optical power is distributed over a very large number of wavelengths, the Kerr effect can cause the following nonlinear optical phenomena:
(1) Self-phase modulation (SPM)—This nonlinear phenomenon is a broadening of the bandwidth of an optical channel due to its own power. This phenomenon impairs transmission by exacerbating dispersion induced ISI, and can cause interchannel crosstalk in WDM systems with close channel spacing.
(2) Cross-phase modulation (XPM)—This phenomenon is a broadening and/or shifting in frequency of an optical channel induced by the intensity of the other channels. Impairments due to XPM are qualitatively similar in effect to SPM, though they may be quantitatively dominant for systems with large channel counts and close channel spacing.
(3) Four-wave mixing (4WM)—This phenomenon describes the interaction of channels at two separate wavelengths, generating power at a third wavelength, which may overlap and interfere with a third data channel. Four-wave mixing is especially problematic in WDM systems with many channels evenly-spaced in frequency. Particular data channels may be overlapped by mixing products from many pairs of other channels. The effect is equivalent to increasing the noise and/or crosstalk in that channel.
As the Kerr nonlinearity operates on the light intensity (optical power per unit area), these nonlinear effects may be mitigated either by using low optical powers or by using fibers with relatively large effective mode field areas (Aeff), i.e., with a core size that is as large as practical but still single mode. The 4WM is a coherent effect, and so may be mitigated by constructing the transmission link from alternating types of NZD as described previously.